In addition to having a function of preventing scattering in the air of toner particles having a size of about 1 μm, the carrier liquid of a liquid toner (liquid developer) has a function of bringing toner particles in a charged, uniformly dispersed state. In development and electrostatic transfer processes, the carrier liquid plays a role for facilitating electrophoresis of toner particles under the action of an electric field.
For example, in a liquid development printer process, a carrier liquid is a component required for storage of toner, conveyance of toner, layer formation, development, and electrostatic transfer. However, during and after the process of fixation on printing medium, the carrier liquid is unnecessary in terms of image quality and the like. For these reasons, volatile, electrically insulative solvents are currently used as carrier liquids of many liquid toners. When a volatile carrier liquid is used, the carrier liquid is volatilized and removed from a toner image through application of heat at the time of fixation. Since a hydrocarbon solvent is usually used as the volatile carrier liquid, in light of influence on the human body, the volatilized carrier liquid must be collected so as to prevent release to the exterior of the apparatus. Thus, a large-scale collection apparatus is required.
In order to cope with firm adhesion of toner to the interior of the apparatus as a result of volatilization of solvent, influence of a volatilized carrier on the human body, and environmental problems induced by the volatilized carrier, liquid toners that use a nonvolatile carrier solvent have been developed. Among them is HVS (High Viscous Silicone-oil) toner.
In a liquid-development apparatus using a nonvolatile carrier liquid, a toner image formed on an intermediate transfer member is heated, and the carrier liquid is removed, whereby the nonvolatile carrier liquid can be effectively removed. Through such removal of the carrier liquid, while wetting of a printing medium and a fixation defect which might otherwise result from the carrier liquid are prevented, a toner image can be transferred and fixed to the printing medium.
FIG. 27 shows a conventional liquid-development electrophotographic apparatus. In the illustrated apparatus, a photoconductor member is charged by means of a charger, and optical exposure of a printing image is effected by an exposure unit so as to form an electrostatic latent image on the surface of the photoconductor member. A developing unit is configured such that a liquid toner is used as developer; the liquid toner is thinly applied to a developing roller; and the developing roller is in contact with the photoconductor member. The electric field force of the electrostatic latent image formed on the surface of the photoconductor member causes toner particles of the liquid toner on the developing roller to adhere to the electrostatic latent image.
The thus-formed toner image on the photoconductor member is transferred to an intermediate transfer member. After transfer of the toner image to the intermediate transfer member, the photoconductor member is destaticized by means of a destaticizer, and then undergoes formation of the next image. The toner image transferred to the intermediate transfer member is transferred to a printing medium. At the time of this transfer, the toner image on the intermediate transfer member is heated so as to be sufficiently melted.
In such a liquid-development electrophotographic apparatus, in order to lessen thermal damage to the photoconductor member, the intermediate transfer member must undergo cooling before coming into contact with the photoconductor member. This requires a large quantity of energy (refer to Japanese Patent Application Laid-Open Nos. 2001-22186 and 2001-305886).
In order to avoid damage to the photoconductor member which would otherwise result from the photoconductor member being heated through contact with the intermediate transfer member which has been heated at the time of transfer to the printing medium, after transfer to the printing medium, the intermediate transfer member must undergo cooling. In order to enable this cycle of heating and cooling, the intermediate transfer member must be of sufficiently large size in order to render time before cooling sufficiently long, resulting in an increase in the size of the apparatus. Also, repeating heating and cooling requires a large quantity of energy.
Also, in the conventional liquid-development electrophotographic apparatus, pressure to be imposed on the printing medium raises a problem. A toner image is transferred from the intermediate transfer member to the printing member by means of electrostatic transfer effected through application of voltage. Since electrostatic transfer is influenced by the electric resistance of the printing medium, it is highly dependent on environmental factors such as ambient temperature and humidity, thereby imposing limitations on environmental specifications of the electrophotographic apparatus.
In order to solve the above problem, there has been employed a melt transfer-and-fixation process in which toner is brought in a molten state so as to attain adhesion, and the molten toner is transferred to a printing medium. Specifically, as shown in FIG. 28, the intermediate transfer member and a backup roller are heated by means of a heater so as to melt a toner image on the intermediate transfer member, and then the molten toner image is transferred to the printing medium through application of pressure effected by the backup roller.
In this case, dependence on environmental factors can be lowered. However, since adhesion of toner is used for transferring a toner image to the printing medium, transfer pressure must be extremely high (1 MPa or higher). This raises the following problem: vibration generated on the intermediate transfer member when the printing medium is nipped in a contact section between the backup roller and the intermediate transfer member is transmitted to the photoconductor member and the developing units, which are drivingly linked to the intermediate transfer member, thereby causing generation of image distortion called shock marks. Also, as a result of subjection to excessive pressure in the contact section between the backup roller and the intermediate transfer member, toner which remains on the intermediate transfer member without being transferred to the printing medium at the time of transfer of a toner image firmly adheres to the surface of the intermediate transfer member; and a cleaning unit encounters difficulty in removing the residual toner.
Furthermore, in the liquid-development electrophotographic apparatus, presence of excess carrier at the time of transfer to the intermediate transfer member or paper affects melting of a toner layer at the time of fixation, and causes a fractural separation of the toner layer at the exit of a nip zone at the time of transfer, with a resultant disturbance of image due to generation of a streaky pattern called riblet (ribs).
Thus, excess carrier liquid must be removed. However, in contrast to the case where a volatile carrier liquid is used, in the case where a nonvolatile, high-viscosity, high-concentration liquid toner is used as developer, a carrier cannot be removed through vaporization. Thus, removal of carrier is performed on the photoconductor member at a position located downstream of a development position and on the intermediate transfer member.
In order to enhance transfer efficiency, Japanese Patent Application Laid-Open (kokai) No. 2001-60046 discloses the technique of increasing adhesion between toner particles and a printing medium through employment of temperature settings represented by the relation “surface temperature of an image bearing member≦glass transition point of toner particles<temperature of a printing medium.”
However, when the surface temperature of an image bearing member is set lower than the glass transition point of toner particles, toner solids tend to hold the carrier, thereby impairing the carrier removal efficiency. As a result, after transfer to a medium, a fixation defect arises.
Similarly, according to Japanese Patent Application Laid-Open (kokai) No. 2001-92199, in order to enhance transfer efficiency, the temperature of an image bearing member and the temperature of a transfer destination member are set higher than the glass transition temperature of a liquid toner.
However, in the case where carrier removal is performed with the surface temperature of the image bearing member being set higher than the glass transition point of toner particles, after sufficient removal of the carrier (in a solid proportion of 50% to 90%), the adhesion between the image bearing member and toner increases. Thus, even when the temperature of the transfer destination member is set higher than the glass transition temperature of toner, transfer efficiency is impaired.
Furthermore, a fixation process in electrophotographic image formation generally employs a fixation process using heating rollers. According to a heat-roller-type fixation process, a printing medium to which a toner image has been transferred in a transfer process passes a nip width which a pair of heat-controlled heating rollers form when they are pressed against each other, whereby thermoplastic toner is heated and melted. This fixation nip zone of the heating rollers simultaneously performs heat transmission to a toner image for melting the toner image, and application of pressure to the toner image for close contact of the toner image with and penetration of the toner image into the printing medium. As a result, final image strength, such as strength of adhesion to the printing medium or resin strength, is developed.
However, in the heat-roller-type fixation process, since toner is heated to a temperature equal to or higher than its melt temperature Tm [° C.], a problem called “high-temperature offset” may occur. The “high-temperature offset” is a phenomenon in which molten toner adheres to a heating roller, because of insufficient toner cohesion caused by the decreased viscosity of the molten toner. According to general measures to cope with the problem, the surface of a heating roller—which comes in direct contact with a toner image—is formed of a fluorine-containing resin coat or silicone rubber of excellent parting performance and is additionally coated with a parting oil typified by silicone oil.
These measures can lower adhesion to a heating roller and thus yield the desired effect to a certain extent, but raise a new problem. For example, when silicone oil serving as a parting oil is applied to the surface of a heating roller, depending on the quantity of application, a printing medium, such as paper, becomes translucent because of wetting, or excessive gloss or glare is imparted to an image, thereby developing a wrong representation of image quality. In some cases, silicone oil itself may hinder melt integration of toner.
FIG. 29 shows a conventional toner fixation unit for use in a full-color electrophotographic apparatus. Referring to FIG. 29, generally, in a full-color electrophotographic apparatus, in order to obtain good color development, toner is completely melted and fixed on a printing medium. In order to completely melt and fix toner on the printing medium, toner and the printing medium are heated to the melting temperature of toner in the fixation nip zone of paired fixation rollers consisting of a heating roller for heating the image side of the printing medium and a backup roller to apply pressure to the printing medium; and molten toner is brought in close contact with the printing medium through application of pressure from the paired fixation rollers. Accordingly, when printing speed increases through attainment of high-speed rotation of paired feed rollers for feeding the printing medium, time for the printing medium to pass through the fixation nip zone is shortened, thereby raising difficulty in raising the temperature of the printing medium.
Also, molten toner exhibits an increase in adhesiveness and thus adheres not only to the printing medium but also to a heating roller (high-temperature offset). This adhesion to a heating roller must be avoided. According to the prior art illustrated in FIG. 29, in order to wipe off adhering toner from the heating roller, a cleaning belt and a cleaning roller are provided. Generally, in order to hinder high-temperature offset of toner to the heating roller, silicone oil having a viscosity of about 50 cSt to 100,000 cSt is applied as a parting agent to the heating roller at all times by means of an oil application roller or the like. This raises another problem of adhesion of a large quantity of silicone oil to the printing medium.
FIG. 30 is a diagram illustrating a toner and printing medium surface temperature history as observed in a fixation nip zone. In FIG. 30, Tg represents glass transition temperature; Tm represents the melting point of the resin component of toner particles; and Toff represents an upper-limit temperature at and below which high-temperature offset does not occur. The cause of high-temperature offset in a heat-roller-type fixation process is as follows. As illustrated in FIG. 30, a toner image on the printing medium is of low temperature at the entrance of the nip zone and is heated through heat transmission from a high-temperature heating roller. Thus, the highest temperature is marked at the exit of the nip zone of the heating roller. At this time, the temperature rises above the high-temperature-offsetless upper limit temperature Toff, thereby causing occurrence of high-temperature offset. As described above, high-temperature offset occurs when the temperature as measured at the exit of the nip zone exceeds Toff. Thus, the general fixation process—in which the temperature as measured at the exit of the nip zone marks a highest value in temperature history—is disadvantageous in terms of high-temperature offset.